I'm new and I'm jumping in here, so my apologies if I'm preaching to the converted on this one. Also I think this also applies to your 'pitfalls' posting. Either way you've got your work cut out!

Implementing a Service Desk has a lot of considerations surrounding headcount, technology and user communication etc., but it doesn't necessarily improve anything unless you're planning to bring in the core processes around it that will support it long term: Incident, Problem and Change Management in some form or other, plus some kind of SLA.

Seeing as the you don't currently have the latter I would suggest setting a very general one to use internally in IT to start with, but try and open some kind of dialogue with the Business over this anyway. If you don't have some form of measurement and tangible metrics you won't know whether it's any better than your old Helpdesk. It is imperative that the Business are involved as early as possible in such developments.

So once you've got a plan for what these deliverables are then we can talk about a project plan.

Sorry this is not immediately what you want but every scenario is different.

what UJ has said is very relevant to what needs to be done. Process is a very important part and should not be over looked.

While I don't have a project plan to show you, here are a few thoughts. What is the current attitudes from your clients towards the Service Desk (SD)? Do they speak highly of the SD, or do they look for ways to circumvent the SD? If they circumvent the SD, then I would look to implement something that would provide quick wins and increase the visibility of the SD.

As the quicks wins get implemented and the SD reputation increases, you should also be working on process along the way. By the time the processes are in place, you would have hopefully changed the view of the SD.

I agree with all of the comments above... The major thing is perception, I always go on about this as this is what rules our reality. The perception that our clients have of us as a whole.... try and find that out. Speak to the stake holders of your business and find out what they think of the SD. Sometimes the smallest things can cloud a perception, the same is true of increasing perception, small but solid improvements and contact can build a good relationship.